Water or moisture penetration into any outside cable plant can cause communications circuits to fail. Outages often occur at the most inopportune time and are costly to locate and repair. The presence of water in the cable may cause a variety of problems including the short circuiting of copper pairs and degradation of mechanical components of the cable. With fiber optic cable, freezing of penetrated water may cause rupture of the fibers and the water may attack and separate the fiber coatings. This may result in increased microbending and attenuation, stress, fatigue, and ultimate failure of the fibers.
A wide range of materials and methods have been employed with varying degrees of success, in an effort to maintain outside cable plant. These include pressurization, the use of double enclosures and encapsulation. All of these techniques are costly and do not provide for the detection of water entry.
In addition, pressurization is ineffective on fiber optic cables while double enclosures are bulky and encapsulating materials are messy to handle.
A modern communication cable is constructed with a waterproof sheath, protecting the cable core. In most cases, the cable sheath incorporates one or more layers of metal armour encased in waterproof layers of plastic, for example, polyethylene. In optical fibre cables, the core usually incorporates a central strength member which may be steel or dielectric. The modern communication cables are usually "filled" with a water-blocking jelly to keep water out. No moisture-absorbant materials are allowed.
While it has been proposed in the past to produce electrical cables such as those described in Alles U.S. Pat. No. 2,056,085 and in Vokey U.S. Pat. No. 4,386,231 which incorporate special moisture-detecting conductors, such cables are undesirable from the point of view of the user because their special construction makes them more costly. The Alles construction is also not acceptable because it incorporates an absorbent material. A further difficulty arises with fibre optic cables, where the small physical size of the cable core makes it unsuitable for wrapping with a moisture-detecting tape such as that described in Vokey. Of even more importance is the fact that using the prior art monitoring techniques, for example that described in McNaughton et al U.S. Pat. No. 4,480,251 only the especially constructed cables can be monitored. This means that the very large quantity of existing conventional cable cannot readily be monitored for moisture penetration. The disclosure of the McNaughton et al patent is incorporated herein by reference.
Another disadvantage of the known cables and systems is the location of the detecting condutors inside the protective sheath of the cable. This means that the presence of moisutre cannot be detected until after it has prenetrated the entire sheath, including the Waterproof plastic layers and cable armour.
The present invention is concerned with the provision of a system for monitoring a communications cable of the conventional type, that is without special moisture-detecting conductors built into it, for moisture penetration. The system is intended to provide an early detection of any moisture penetration or physical damage to the cable armour.
The present invention provides a moisture-detecting system for monitoring moisture penetration in a conventional communications cable, provided that the cable is equipped with at least one conductive armour layer. Where the cable has a conductive component of this type, the invention can use it to communicate between a central office terminal, a terminating resistor and remote sensors in splice enclosures along the cable. A DC current through the conductor, the terminating resistor and a ground return path provide for end-to-end monitoring of the cable. An allowable current "window" is established and if the current exceeds a pre-set value, this indicates a short in the system, such as through moisture penetration. A current below a minimum value indicates an open circuit in the conductor.
Using this technique, moisture penetration can be monitored in conventional communications cable without special moisture-detection conductors.
Because the armour layer is within the protective sheath, just inside the outer plastic jacket, moisture penetration is detected much earlier than with the prior art, which required moisture penetration through the entire sheath to the core before it could be sensed. Physical damage to the armour layer is also monitored, which can occur without moisture penetration. By using a negative potential line signal on the cable armour, the armour is cathodically protected so that corrosion is inhibited even in the event of moisture penetration of the jacket.
The present invention aims at an improved system of this type.